[Correction of leg deformities. Definition, estimation and realignment of axis deviation and misalignment]

The operative realignment of mechanical axis deviations (MAD) is necessary to prevent early joint degeneration and to reach normal load-bearing. Modern types of external fixation systems allow an alignment of the mechanical axis to exact degrees. Predominantly this are corrections of angular deformities. In some cases the analysis of the MAD does not show any angular deformity, but a parallel staggering of the mechanical axis lines of a bone. Such parallel staggering of the mechanical axis lines is defined as a translation deformity of the bone. In combined deformities with angulation and translation the centre of deformity can be established proximal or distal out of the limit of the bone. In translation deformities the realignment of the mechanical axis requires a parallel re-staggering made by a translation-osteotomy or by a counter-angulated double osteotomy. In complex deformities with angulation and translation, the translation requires a separate corrective planning. By using external fixator systems to perform acute or progressive corrective osteotomies the position of the Axis of Correction of Angulation (ACA) in relation to the n-CORA of the deformity has to be considered. If ACA position is not conform with the n-CORA position relevant geometric effects in relation of length and translation occur. These geometric effects to corrective osteotomies can be calculated by using simple trigonometric formulas or graphical methods. Possibilities to compensate translation and length effects are shown by using unilateral external fixator systems.